1. Field of the Invention
The present invention relates to a stable modified metal oxide sol which contains particles comprising colloidal particles of a metal oxide as nuclei and a coating material consisting of an alkali salt of an acidic oxide, colloidal particles of an acidic oxide or a mixture thereof, coated on the surface of the colloidal particles as nuclei, and a process for producing it.
The colloid of the present invention is useful as a component for a hard coating agent to be applied to the surface of plastic lenses.
Further, the present invention relates to a coating composition which provides a coating film having excellent warm water resistance and having no decrease in weather resistance and light resistance even when a vapor deposition film (such as an antireflection film) of an inorganic oxide is provided on the coating film, and an optical element employing the coating composition.
2. Discussion of the Background
In order to improve the surface of plastic lenses which became used widely in recent years, as a component for a hard coating agent to be applied to said surface, sols of a metal oxide having a high refractive index and having a good compatibility with the hard coating agent have been used.
For example, a stable sol of tungstic oxide alone has not been known yet, but a sol having a WO3:SiO2:M2O molar ratio (wherein M is an alkali metal atom or an ammonium group) of 4 to 15:2 to 5:1, obtained by addition of a silicate, has been proposed in JP-A-54-52686.
JP-B-50-40119 proposes a silicate-stanate composite sol having a molar ratio of Si:Sn of 2 to 1000:1.
JP-B-63-37142 discloses a hard coating agent which contains particles of an oxide of a metal such as Al, Ti, Zr, Sn or Sb, having particle diameters of from 1 to 300 nm.
Further, JP-A-3-217230 proposes a stable sol containing colloidal particles of a modified metal oxide having particle diameters of from 4.5 to 60 nm, which comprise colloidal particles of an oxide of a metal with a valence of 3, 4 or 5, having particle diameters of from 4 to 50 nm, as nuclei, and colloidal particles of a tungstic oxide-stannic oxide composite having a WO3/SnO2 weight ratio of from 0.5 to 100 and having particle diameters of from 2 to 7 nm, coated on the surface of the colloidal particles as nuclei, wherein the content of the total metal oxides is from 2 to 50 wt %.
Further, JP-A-6-24746 proposes a stable sol of a modified SnO2—ZrO2 composite which contains particles comprising colloidal particles of a SnO2—ZrO2 composite having a weight ratio of ZrO2/SnO2 of from 0.02 to 1.0 and having particle diameters of from 4 to 50 nm, as nuclei, and colloidal particles of a WO3—SnO2 composite having a WO3/SnO2 weight ratio of from 0.5 to 100 and having particle diameters of from 2 to 7 nm, coated on the surface of the colloidal particles as nuclei.
Still further, JP-A-10-310429 proposes a stable sol of a TiO2—ZrO2—SnO2 composite oxide.
Plastic molded products are used in a large quantity by virtue of their advantageous features such as light weight, good processability and high impact resistance. On the other hand, they have drawbacks that the hardness is inadequate, and thus they are susceptible to scratching, they are likely to be eroded by a solvent, they are likely to be electrified and adsorb a dust, and the heat resistance is inadequate. Thus, as compared with inorganic glass molded products, they were practically inferior for use as lenses for eyeglasses or window materials. Accordingly, it has been proposed to apply a protective coating to a plastic molded product. Many compositions have been proposed as coating compositions to be used for such a protective coating. For example, JP-A-52-11261 proposes to use “a coating composition containing an organic silicon compound or its hydrolyzate as the main component (resin component or coating film-forming component)” for eyeglass lenses, which was expected to provide a coating film as hard as an inorganic product. However, this coating composition still does not provide adequate scratch resistance. Accordingly, JP-A-53-111336 proposes one having colloidal silica particles added to the above coating composition, which is used practically for eyeglass lenses.
Heretofore, plastic lenses for eyeglasses have been produced by casting diethylene glycol bisallyl carbonate in a monomer state, followed by polymerization. The lenses produced in such a manner have a refractive index of about 1.50, which is low as compared with the refractive index of about 1.52 of glass lenses, and in the case of lenses for short sighted, there is a problem that the peripheral thickness has to be increased. Accordingly, in recent years, there has been development of monomers having higher refractive indices than the diethylene glycol bisallyl carbonate. For example, resin materials having high refractive indices are proposed, for example, in JP-A-55-13747, JP-A-56-166214, JP-A-57-23611, JP-A-57-54901, JP-A-59-133211, JP-A-60-199016 and JP-A-64-54021.
For lenses made of such resins having high refractive indices, JP-A-62-151801 and JP-A-63-275682 propose a method of using a colloidal dispersion of fine particles of an oxide of a metal such as Sb or Ti, for a coating material.
If such a conventional metal oxide sol, particularly a cationic metal oxide sol, is used as a component for a hard coating agent, not only the stability of the obtained hard coating agent tends to be insufficient, but also e.g. transparency, adhesion and weather resistance of the cured coating of the hard coating agent tend to be insufficient. Further, in a case where a Sb2O5 sol is used as a component for a hard coating agent, the refractive index of the cured coating will no longer increase adequately with this Sb2O5 sol if the refractive index of the plastic substrate for a lens is at least 1.60, since the refractive index of Sb2O5 is a level of from 1.65 to 1.70.
The above sol of tungstic oxide as disclosed in JP-A-54-52686 is obtained by adding a silicate to an aqueous solution of tungstic oxide obtainable by subjecting an aqueous solution of a tungstate to cation exchange. However, the sol is stable only in a strong acidic condition, and its effect to increase the refractive index of the coating film is small when used as a component for a hard coating agent.
The above silicate-stannate composite sol as disclosed in JP-B-50-40119 is obtained by subjecting a mixed aqueous solution of an alkali silicate and an alkali stannate to cation exchange. However, its effect to increase the refractive index of the coating film is also small when used as a component for a hard coating agent.
The coating composition having a silica sol added thereto, has a problem that the coating film is likely to have interference fringes which impair the appearance of the lenses. Further, in lenses, an antireflection film (composed of a multilayer structure film comprising thin films of inorganic oxides, based on an optical interference theory) is formed in many cases, on the coating film. In such a case, the antireflection film tends to exhibit, for example, a reflection color of extremely pale green, and this reflection color changes depending upon the position on the lens surface to form flecking.
A coating composition prepared by using a titanium oxide sol has a problem that the titanium oxide sol has a low compatibility with a silane coupling agent or its hydrolyzate, the stability tends to be low, and the coating layer formed by this coating composition tends to be poor in water resistance and tends to be blued by irradiation with ultraviolet rays.